The magnetic devices used to read and write data from the media on a hard disk are called sliders or heads. The previous generation of heads used a single inductive head for both the reading and writing, but such technology could not provide the necessary performance improvements for higher capacity hard disks in high volume production.
Winchester style sliders having thin film, magneto-resistive (MR), giant magneto-resistive (GMR), spin valve, or other types are now being used in magnetic hard disk storage systems to read information magnetically encoded in the magnetic media of the hard disk, with MR elements being the most popular. GMR heads are emerging quickly. A magnetic field extending from magnetic media caused by the spinning of the disk directly modulates the resistivity of the MR element. The change in resistance of the MR element normally is detected by passing a sense current through the MR element and then measuring the changes in voltage across the MR element. The resulting signal is used to recover the digital magnetically encoded information.
Read/write heads are produced by forming the separate read and write elements on a ceramic wafer in a deposition process somewhat similar to that used in the semiconductor industry. The wafer is cut into rows and the slider surfaces are then machined and lapped for proper magnetic and flying height characteristics as described in U.S. Pat. Nos. 5,607,340 and 5,620,356 both by Lackey et al. Tolerances are in the millionths of an inch and are getting tighter as areal densities (the storage bits per unit area) increase. The top surface of the wafer eventually becomes the back surface (trailing end) of the slider, perpendicular to the slider surface (air bearing surface) of the head that forms an air bearing with the media. The electrical resistance of the magneto-resistive material changes when a magnetic field sweeps there through. Normally, a MR head includes a MR stripe having upper and lower sides parallel to the spinning disk media, and conductors that overlay the ends of the stripe at right angles thereto. The conductors define the ends of the stripe and provide the electrical path for the sense current that is used to read the bits of magnetically information. The bits are recorded on the magnetic media by a separate inductive element. The inductive element is formed on the back surface of the head during the wafer process spaced from the MR element.
The change in resistance in a MR element occurs because the magnetic field causes the impedance vector of the material to rotate from a pure resistance, which has the effect of changing the resistance portion of the impedance vector. The effect in the present generation MR elements results in a maximum change in resistance, from 2 to 10%. In the next generations of multi-layer elements, each provide significant improvement, that is the newly available giant MR elements produce a .DELTA.R of about 10 to 30% and the planned colossus MR elements are expected to produce a .DELTA.R of over 30%. The more an MR element changes its resistance when exposed to a magnetic field, the smaller the MR sensor element can be, allowing narrower tracks and smaller magnetized areas, so that more data can be stored per unit area of magnetic media.
The signal to noise ratio of a MR element varies with ratio between the resistance, R, of the stripe and the change in resistance, .DELTA.R, of the element when subjected to the sweeping magnetic field. The thickness and to a lesser extent, the composition of a stripe are difficult to precisely control during the wafer fabrication process and therefore a precision lapping process that removes material from the flying surface of the slider is used to trim the height of the stripe to obtain maximum signal to noise ratio. If the stripe is too tall, the resistance is to low with respect to .DELTA.R and the voltage variations due to passing magnetic fields are too low, while if the stripe is too short, the resistance is too high, and the voltage variations due to passing magnetic fields again are too low. In the next generation of heads for drives with even higher areal densities (number of bits per square inch) requiring smaller MR elements, stripe height control to maximize signal output will become ever more critical, requiring lapping to magnetic performance and control on the order of a millionth of an inch. In addition, the stripe height lap and a final crown lap need to be combined since stripe height is reduced by the final crown lap.
MR elements are constructed by laying down thin stripes of MR material using wafer fabrication techniques similar to those developed in the semiconductor industry. The wafer is then sliced so that the MR stripes are positioned adjacent what will become the slider air bearing surface along what will become the trailing or back edge of the slider. Two conductors are formed over each end of the stripes so that the changing resistances due to magnetic fields impinging therein can be measured by a sensing current fed there across.
The most common control approach for lapping uses magneto-resistive electrical lapping guides (MR ELGs) that are formed at intervals along each row of MR elements. Generally MR ELGs are long MR elements with separate connections to the control systems for the lapping machines. In order to find the proper relationship between the stripe height and the measured resistance, it is necessary to calculate the "sheet resistance" of the MR element by finding the sheet resistance of the surrounding MR ELGs. There are many circuit designs for performing this type of calibration of the sheet resistance.
Unfortunately, the resistivity of the MR film varies over each wafer and more particularly over the length of a row of elements on the wafer. Therefore, the resistivity of MR elements distant from a MR ELG and the MR ELG may be different, creating an electrical offset error from head to head and from MR element and the MR ERG. Also, feedback from a MR ELG, which is physically offset from the MR element whose height it is trying to control, creates a physical offset error. This may seem minor, but if the distance between a MR ELG and the MR element whose height its is controlling is 0.008 inches and the desired control is 1 microinch, this is a ratio of 1 to 8,000. Some data scatter is also attributable to imprecise formation of the MR stripes.
One solution for variations in sheet resistivity and stripe variations suggested in the past, was to measure the resistance of an MR element as its height is being trimmed during the lapping operation. With prior technology, direct measurement has been only marginally acceptable. Since the MR elements are microscopic, there is often a large error between actual stripe height and measured resistance. There also is a "blurring" of the contact between the ends of the MR element and the conductors. Since the MR element is short, this blurring becomes a significant percentage. Separate MR ELGs are typically 10 to 20 times longer than the MR element, which minimizes this "blurring" error. Also, to sense the resistance of MR elements directly requires electrical connections and disk drive manufacturers typically do not want wire bonding marks that result from the bonded connections nor probe card marks, present on the MR element bond pads, because such can adversely affect the reliability of new wire bonds or pressure connections when pressure contact pads are employed.
Current fabrication techniques cannot maintain the needed control of sheet resistance so the width of the stripe is critical to get the optimal response from the MR element, which is a function of element resistance and .DELTA.R resistance due to the impingement of a magnetic field. Therefore, a lapping operation of the slider air bearing surface has been used to adjust the width of the MR strip to an accuracy of several millionths of an inch with processes, machines, and devices such as shown and described in U.S. Pat. Nos. 5,607,340 and 5,620,356, both by Lackey et al.
During head production, batch fabrication is employed whereby a plurality of transducers are sliced from a ceramic wafer in a row and bonded onto a row bending tool for stripe height lapping. Row bending tools are commonly constructed from ceramic or steel in a configuration of flexures that allow forces applied to a row bending tool to deform the attached row in up to a fourth order curve in a single plane. During the manufacture of the sliders, this allows a plurality of MR transducers to have their stripe height to be precisely lapped to achieve a desired stripe height at which optimum data signal processing can be realized. The stripe height of all the transducers made during a production run for use with a data storage product must be maintained within a defined limited tolerance.
The process steps performed on the wafer, generate residual stresses, which can cause the rows to bend when they are sliced away from the rest of the wafer, a condition known as "row bow". Although the level of stress can be reduced through care in the wafer fabrication process, it can not be eliminated. Also some manufacturers have processes where reduction of residual stress is not stressed as much as others. Although a curved row theoretically can be straightened for lapping by bonding it to a row bending tool, the stresses are not always uniform across a row, resulting in kinking of the row during bending in the lapping operation. The result is a wide variation in stripe heights across the row after the lapping operation. This variation in stripe height affects ultimate process yields as MR elements get smaller. As a result, MR sensors can not be properly lapped with high yields at the very close tolerances needed when sliders below 50% (&gt;2.05 mm length.times.1.6 mm width.times.0.43 mm thick), that is 50% of an early initial slider standard of 4.02 mm length.times.3.2 mm width.times.0.86 mm thick, are constructed. Also, such sliders present such a small surface opposite the surface to be lapped that they are difficult to mount to a row bending tool and lap to the desired slider surface shape.
Prior attempts to correct for ceramic bar or slider bar distortion are disclosed in U.S. Pat. Nos. 5,117,589 and 5,203,119, 5,607,340, and 5,607,340. However, none are totally satisfactory, when extraordinary care is not used in the wafer processing to minimize residual stresses.
Therefore, a long-standing need has existed to provide an apparatus and method to relieve residual stresses in a row of sliders and to accurately mount it on a row bending apparatus so that MR sensor stripe height on a plurality of sliders in the row can be accurately controlled during lapping by accurately bending the row or varying the lapping pressure of individual heads.
Also, there has been a long-standing need for handling the individual heads during the slider fabrication operation. The bonding on the row during lapping is just one of a plurality of bonding and debonding operations. As the row and the heads become smaller and more fragile, there is a yield loss during each bonding and debonding operation. After the row is bonded to the row carrier after slicing, the row carrier becomes "smaller and more fragile, there is a yield loss during each bonding and debonding operation."